The current trend in the design of wind turbines is to increase the length of the blades in order to boost the annual energy production, trying to achieve a compromise between lightness and stiffness.
For this purpose, in the blade design stage the most adequate characteristics for each longitudinal component forming the blade are determined. Those longitudinal components are:                an outer shell which provides the aerodynamic geometry of the blade, generally formed by two valves bonded together,        beams adhered to the shells (or partially embedded in them) which provide the required structural stiffness.        shearwebs which connect the beams to each other and transfer the shear loads.        
In order to achieve light and enough stiff blades, polymeric materials are used in the design including fiber glass and/or fiber carbon fabrics embedded in a polymeric matrix, including in some locations wooden or plastic foam cores.
Generally, the manufacturing process of a longitudinal component of a wind turbine blade consists on arranging a number of layers of polymer fiber fabrics on a mould, stacked one above the other, following a stacking sequence previously established. The stacking sequence determines the number of layers of polymer fiber fabrics and the orientation of the fibers in each layer with respect to a reference direction which is usually the longitudinal direction of the blade. To increase the stiffness and/or enhance the buckling behavior of the longitudinal component, lightweight cores typically made of plastic foams or wood are arranged in certain parts between the layers of fabric. To guarantee the bonding of the different layers of fiber fabric among them and with the cores, a polymeric material, called matrix, is used.
The application of this matrix is performed by any of the usual procedures employed in the manufacturing of composite materials (resin infusion, RTM in its different variants, pultrusion, etc.) which in turn involve the use of different auxiliary elements to make the manufacturing process easier. In other cases, the fiber is presented as fabrics total or partially impregnated of said matrix (pre-pregs).
Usually the fiber orientations of the fabrics are selected depending on the longitudinal component of the blade and the type of stresses that it is subjected to. The most common orientations are 0°, ±45° and 90° with re spect to the longitudinal direction of the blade. Furthermore, the thickness of the composites (i.e. the number of layers of the composite formed by resin and fabric) used in each zone of the longitudinal components of the blade is selected depending on the magnitude of said predominant stresses.
Fabrics are formed using the most appropriate fiber orientation (or the most appropriate fiber orientations) for each longitudinal component. The fabrics may comprise fibers with different orientations with respect to a reference direction. Generally the fabrics comprise fibers with 1, 2 or 3 with the following standard combinations:                uniaxial at 0°        biaxial at +/−45°        triaxial at 0° and +/−45°        
These fabrics are subsequently placed in the mould for manufacturing the blade, extending all along each longitudinal component.
Thus, depending on the type of stresses that each longitudinal component withstands, one or another type of fabric, or even combinations of several layers of different types of fabrics are used. Furthermore, depending on the magnitude of stresses, the number of layers for each zone of the longitudinal component are selected.
In state of the art configurations, uniaxial fibers at 0° on beams and biaxial fibers at +/−45° or triaxial 0° and +/−45° on shearwebs and the outer shell are normally used.
The applicant knows the existence of precedents which describe the use of biaxial or triaxial fabrics with angles different from 45°. Among them, the article “Innovative design procedures for large-scale wind turbine blades (JEC Composites Magazine. N° 70 January-February 2012)” is known, where it is proposed to use fabrics with fiber orientations different from 45° in certain components of the blade according to the relationship between normal and shear stresses in each of these components, wherein more specifically it is stated that the shell of the blade is completely manufactured from triaxial fiber fabrics with orientations 0°, +/−25°.
However, in the previous precedent a single fiber orientation in uniaxial fabrics or a single combination of fiber orientations in biaxial or triaxial fabrics is used for the entire longitudinal component of the blade, so that the same fiber orientation (or the same fiber orientations) is used within the entire longitudinal component.